The present invention is directed to a single piece trigger and knock off mechanism. Trigger and knock off mechanisms are well-known in the time recorder field wherein a card or other document is to be imprinted with data such as attendance times and the like. The trigger and knock off mechanism controls a switch operable card printer which includes a print hammer for printing the data on the card. During a print operation, the print hammer strikes the card, pressing the card against an inked ribbon and the characters or numerals on one or more typewheels, and then rebounds from the card.
The card printer includes a switch operated solenoid which drives the print hammer. The trigger and knock off mechanism must close the switch to fire or trigger the solenoid when a card is inserted in the time recorder. The mechanism must also open the switch to de-energize or knock off the solenoid just before the print hammer strikes the card. By "knocking off" the solenoid is meant deenergizing the solenoid so that the print hammer is driven by the solenoid only during part of its flight. Just before and after striking the card, the print hammer must move freely under its own inertia so that the hammer can strike the card in free flight and then rebound in free flight without striking the card a second time.
Conventionally, trigger and knock off mechanisms comprise multiple interrelated parts which enhance manufacturing costs, cost of assembly and susceptability to wear.
An object of the present invention is to provide a trigger and knock off mechanism which is a single integral unit, simplifying manufacture, installation and repair or replacement.
Another object of the invention is to provide a trigger and knock off mechanism which operates reliably while experiencing relatively little wear.
Other objects and advantages of the invention appear hereinafter.